This invention relates generally to control of an internal combustion engine equipped with exhaust gas recirculation system, and more specifically to apparatus for controlling the amount of recirculated exhaust gasses and air/fuel ratio of an air/fuel mixture supplied to an internal combustion engine.
Exhaust gas recirculation, which is known as EGR, is used for reducing the concentration of nitrogen oxides (NOx) included in exhaust gasses from an internal combustion engine. Conventionally, to ensure the accuracy of the EGR control, a valve is controlled electronically in order to control the amount of recirculated exhaust gasses.
In such a conventional EGR system, the amount of recirculated exhaust gasses is estimated by detecting an opening degree of an EGR valve. When using such an EGR system for a long period of time, dirty particles, such as carbon and unburnt substances, are attached to the inner surfaces of the EGR valve and EGR passage. Therefore, the relationship between the opening degree of the EGR valve and the amount of recirculated gasses is disturbed raising a problem that the initial object of estimating the recirculation is not achieved. One approach for solving this problem is to measure the amount of recirculated gasses so as to control the same. One example of a conventional arrangement according to such approach is a technique of estimating the amount of EGR by ascertainining the amount of fresh air with an airflow meter being provided to an intake manifold so as to measure the amount of fresh air even during EGR operation. This technique is disclosed in Japanese patent provisional publication No. 57-44760. Another example is a technique of estimating the amount of fresh air using total intake amount of an internal combustion engine and a sensor provided to an EGR valve. This is disclosed in Japanese patent provisional publication No. 57-83641.
Such conventional techniques, however, only estimate the amount of fresh air or the amount of EGR by detecting one of them where their measuring system is provided centering around the intake pipe of the internal combustion engine. Therefore, true state of air-fuel mixing within the combustion chambers of the internal combustion engine cannot be detected. As a result, air/fuel ratio control carried out through computation of fuel injection amount for obtaining optimal air/fuel ratio is apt to suffer from errors. Furthermore, the above-mentioned first convetional technique requires an airflow meter which introduces resistance to airflow within the intake system. In addition, since the above-mentioned second conventional technique employs a sensor installed in the EGR valve, the sensor is apt to suffer from deterioration due to dirty particles in the exhaust gasses, while there arises a problem relating to maintenance of the sensor. For these reasons, conventional EGR systems are desired to be improved.